If your model feels sluggish while running, it is possible that the battery pack is not properly charged. Ensure that the battery is charging completely.
It may be possible that the motor is dirty or damaged. Give the motor a thorough cleaning, re-oil (See Re-Oiling Motor Bushings support article), and then install it back onto the motor mount. If the motor is damaged, discontinue use and replace the motor. It is also possible that the motor may be worn out. If the battery is charged, gearing is proper, and the drivetrain is bind free, then the motor may have reached the end of its useful life. Replace with a new motor if necessary.
Inspect the drive train to see if it is binding. Remove the motor's pinion gear and roll the vehicle on a flat surface to check the drive train for binding. (See Drive Train Binding support article for more information)
The electronics in your vehicle may be getting too hot. Let the model cool down between runs.
Check the ESC to see if it is in Training Mode. XL-2.5, XL-5, XL-10, VXL, and EVX 2 speed controls feature training mode which reduces the models power to 50%. (See Programming Your Traxxas ESC support article).
Inspect the models gearing to see if it is overgeared. Use a smaller motor pinion to lessen the load on the motor and increase acceleration.
An axle drive pin could be missing. Inspect the axles and check for worn or missing drive pins. If a drive pin is missing, the wheel can be secured to the driveshaft, but the power will not be transferred to the wheel, resulting in little or no forward/reverse motion. (See Installing Axle Drive Pins)
There may be a poor battery or motor connection. Inspect the battery connectors and motor connectors for poor connections, damaged solder joints, or damaged connectors. Poor connections restrict power transfer. Repair or replace as needed.
The ESC may be in LiPo Mode (if equipped). LiPo mode reduces power to the motor at a certain voltage, preventing the battery from being over-discharged. Stop running the model when there is a noticeable drop in power, let the battery rest and cool, then recharge your LiPo battery. If using NiMh batteries, you can turn off the low voltage detection for proper operation.
For older models, the resistor or speed control could be burned out (for mechanical speed controls only). Visually inspect the resistor - if it's cracked or discolored replace it with a new one. If the resistor looks fine, you'll need to replace the mechanical speed control. The mechanical speed control includes a new resistor just in case.